Hybrid inorganic-organic composite for use as an interlayer dielectric

ABSTRACT

Three-phase composite materials system having a low dielectric constant and physico-chemical properties suitable for IC fabrication conditions, and a method for making such materials, are disclosed. The three-phase composite material includes an organic phase, an inorganic phase and a void phase. The organic phase is in the form of an organic polymer matrix, the void phase is represented by microporosity present in the matrix, and the inorganic phase is implemented as inorganic particles that are coupled, via a coupling agent, to the organic matrix. The low dielectric constant of the composite is attributable to the microporous organic polymer matrix. The inorganic particles are responsible, at least in part, for providing thermal stability and other required physico-chemical properties to the composite.

STATEMENT OF RELATED CASES

This application is a continuation-in-part of application Ser. No.08/641,856, filed on May 2, 1996, now U.S. Pat. No. 5,739,180.

FIELD OF THE INVENTION

The present invention relates to a materials system for use as aninterlayer dielectric.

BACKGROUND OF THE INVENTION

The semiconductor industry is moving toward increasing devicecomplexity, requiring shrinking geometric dimensions and highercomponent integration with greater densities and more circuit layers.For example, while current generations of integrated circuit devices(ICs) have interconnect line widths down to about 0.35 microns, suchline widths are scheduled to decrease to 0.25, 0.18, 0.13, 0.10 and sub0.10 microns by 1998, 2001, 2004, 2007 and 2010, respectively. See, "TheNational Technology Roadmap for Semiconductors," Semiconductor IndustryAssociation, pp. 1-3, 1994.

Current generations of ICs rely on silicon dioxide as the interlayerdielectric (ILD). A silicon dioxide-based ILD is usually formed viaspin-on glass processes, or more typically from some derivative ofplasma or chemical vapor deposition. While suitable for interconnectline widths of about 0.35 microns, the relatively high dielectricconstant of silicon dioxide, which is generally in the range of about3.9 to 5 depending on processing conditions, renders it unsuitable foruse as an ILD at the aforementioned smaller line widths. Specifically,with silicon dioxide-based ILDs, capacitance increases to a level suchthat unacceptable RC (interconnect) delays and increased cross talkresult, adversely impacting device speed and degree of powerdissipation.

Fluorinated oxides provide an immediate near-term solution fornext-generation devices, i.e., 0.25 micron line width. Such fluorinatedoxides can be synthesized with dielectric constants in the range of 3.2to 3.5. See, Laxman, "Low .di-elect cons. Dielectrics: CVD FluorinatedSilicon Oxides," Semiconductor Int'l., p. 71, May 1995.

The aforementioned conventional interlayer dielectric chemistries may,however, be unsuitable for use in devices requiring interconnect linewidths of 0.18 microns and less. A shift to new types of insulatingmaterials with sub-3 dielectric constants may be required. To that end,candidate low-dielectric constant organic materials are being developed.

One class of candidate low-dielectric constant organic materials areorganic polymers, some of which have a dielectric constant less than 3.Hendricks, "Organic Polymers for IC Intermetal Dielectric Applications,"Solid State Tech., July 1995. Incorporating fluorine into such organicpolymers is known to further lower the dielectric constant.

Most organic polymers do not, however, possess the physico-chemicalproperties required for IC applications, particularly thermal stability.Material characteristics required for interconnect technology other thanlow dielectric constant are well-known and include high thermalstability (sufficient to withstand back-end IC fabrication temperatureswithin the range of 400-500° C.), relatively high resistance todegradation by, or reaction with, chemicals to which the candidatematerial will be exposed during device fabrication, low gas permeabilityand moisture absorption, low coefficient of thermal expansion, hightensile modulus, high etch selectivity and the like. Few organicpolymers are stable at temperatures greater than 350° C.; suchproperties are more typical of oxides and similar inorganics. Hence thecurrent predominant usage of such inorganic materials systems for thisapplication.

Recently, organic-inorganic hybrid systems have been proposed for use asinterlayer dielectrics. One such system is Chemat-B by ChematTechnology, Northridge, Calif. The Chemat-B system involves depositingan organic-inorganic material formulation with subsequent thermaldecomposition of the organic component, which supposedly yields amicroporous inorganic system. The organic constituent is therefore notretained in the final material structure and the ultimate systemconsists of an inorganic framework with discontinuous dispersed voidspace resulting from the decomposed organic. Chemat-B is reported tohave a sub-3.0 dielectric constant. See, 1997 Proceedings, Dielectricsfor ULSI Multilevel Interconnection Conference (DUMIC), Library ofCongress No. 89-644090, pp. 93-97, 1997 ISMIC-222D/97/0295.

Miller et al. of IBM have reported on a nanophase-separatedinorganic-organic hybrid composition having a sub-3.0 dielectricconstant prepared from reactively-functionalized poly(amic esters) andoligomeric silsesquioxanes. See 1997 Proceedings, DUMIC, Library ofCongress No. 89-644090, pp. 295-302, 1997 ISMIC-222D/97/0295.Silsequioxanes are organically-modified inorganic glasses having a lowerdielectric constant, e.g., in the range of about 2.7-3.2, thanconventional inorganic silicate glasses, which have a dielectricconstant in the range of about 3.9-5.0. While having an advantageouslylower dielectric constant than conventional glasses, such materialstypically have poor mechanical properties. In particular, silsequioxanesexperience crack formation during processing. Miller et al. address thatproblem by "rubber-toughening" the virgin silsequioxane material systemsby incorporating a small amount, e.g., about 0-20 weight percent, of anorganic polymeric substituent such as a polyimide.

The material system of Miller et al. may be characterized as a glassyinorganic material with a minor fraction of included organic. Since theincluded organic represents a small portion of the composite system, theresultant dielectric constant is controlled by the inorganic matrix,which is the majority phase. As such, it is unlikely that such a systemcould achieve a dielectric constant much less than that of the dominantinorganic phase, i.e., approximately 3.

Although various systems have been proposed, there remains a need for amaterial having a suitably low dielectric constant and appropriatephysico-chemical properties for use as an interlayer dielectric infuture generations of IC devices.

SUMMARY OF THE INVENTION

A hybrid inorganic-organic composite (IOC) materials system useful as aninterlayer dielectric having utility in microelectronics, and a methodfor its synthesis, are disclosed. The present materials system ischaracterized by a low dielectric constant and as havingphysico-chemical properties suitable for IC fabrication conditions.Interlayer dielectric materials formed according to the presentinvention provide advantages over conventional dielectric materials,especially when used to form interconnect lines having sub-0.25 microninterconnect line widths.

The present interlayer dielectric materials system includes an organicphase in the form of a polymer matrix and an inorganic phase linkedthereto. The inorganic phase and the organic phase are generated fromrespective inorganic phase and organic phase precursors. The organicphase is formed in the presence of a microporosity-imparting agent, sothat, when formed, the organic phase includes void space or pores. Suchpores advantageously lower the dielectric constant of the interlayerdielectric materials system. The present materials system is thus athree-phase system. Additionally, it is preferred that at least one ofeither the inorganic phase or organic phase contains fluorine, known forreducing the dielectric constant of materials systems.

According to the present method, an interlayer dielectric materialssystem is formed by coupling the inorganic phase and the organic phase.In a first embodiment, the aforementioned coupling is promoted via theuse of a coupling agent. The coupling agent functions as a bridge thatlinks the inorganic and organic phases. The link is formed via adual-coupling mechanism wherein a group of the coupling agent forms alink with the inorganic phase and another group of the coupling agentforms a link with the organic phase. In a second embodiment, thecoupling agent is used to form the inorganic phase, thereby providingthe inorganic phase with the group capable of linking with the organicphase. Thus, in the second embodiment, additional coupling agent is notrequired to couple the inorganic and organic phases.

For use as an interlayer dielectric, the present materials system isdeposited on a silicon wafer or other substrate. In its final form,however, the interlayer dielectric, i.e., the hybrid IOC materialssystem, is not amenable to typical deposition methods. It is thereforeadvantageous to deposit a three-phase composite precursor of the finalinterlayer dielectric material on a substrate.

In presently preferred embodiments, the present interlayer dielectricmaterials system is "spun on" to a substrate, etc., using well knownspin-coating techniques. In some embodiments, the precursor formulationcontains the inorganic phase or a precursor thereof, a coupling agent, amicroporosity-imparting agent and the organic component precursor. Insome of the aformentioned embodiments, the coupling agent is coupled tothe inorganic phase or its precursor before depositing the precursorformulation on a substrate. In other of the aforementioned embodiments,such coupling to the inorganic phase or its precursor is initiated afterthe formulation has been deposited on a substrate.

After depositing the precursor formulation, additional reactions areinitiated to form the final hybrid-IOC material. In particular, theorganic phase precursor is polymerized thereby forming the organicmatrix to which the inorganic phase is coupled. The organic phaseprecursor is polymerized only after the precursor formulation isdeposited on a substrate. As the organic phase is formed, it becomesmicroporous through the action of the microporosity-imparting agent.

The present method is quite flexible in terms of the ordering of thesynthesis steps. For example, as noted above, the coupling agent may becoupled to the inorganic phase, or its precursor either before or afteradding the organic phase precursor and microporosity-generating agentthereto. Additionally, as mentioned above, the coupling agent may becoupled to the inorganic phase either before or after depositing theformulation on to a wafer. Also, the coupling agent may be coupled tothe organic phase precursor before it is coupled to the inorganic phaseor its precursor. These and other permutations of a method according tothe present invention are described in more detail below under theDetailed Description. Notwithstanding the aforementioned flexibility,and regardless of other reactions and associations taking place, theorganic phase precursor remains substantially unpolymerized until afterthe precursor formulation is deposited on a substrate.

BRIEF DESCRIPTION OF THE DRAWING

Further features of the invention will become more apparent from thefollowing detailed description of specific embodiments thereof when readin conjunction with the accompanying Figures in which:

FIG. 1 is a flow diagram of a method according to the present inventionfor forming an interlayer dielectric material;

FIG. 2 is a flow diagram showing an exemplary embodiment of a method forforming a spinnable precursor formulation;

FIG. 3 is a schematic illustration of a first embodiment of an inorganiccomponent possessing an organic functionality; and

FIG. 4 is a schematic illustration of a second embodiment of aninorganic component possessing an organic functionality.

DETAILED DESCRIPTION

The present invention is directed toward materials useful as aninterlayer dielectric having utility in microelectronics applications,and a method for their synthesis. Such materials are formable to have adielectric constant less than 3, and to possess the physico-chemicalproperties required for an interlayer dielectric. Regarding suchproperties, an interlayer dielectric should have thermal stability to atleast about 450° C.; relatively high resistance to degradation by, orreaction with, chemicals to which the candidate material will be exposedduring device fabrication, low gas permeability; low moistureadsorption; low coefficient of thermal expansion; high tensile modulus;high etch selectivity and the like. See Murarka, "Low DielectricConstant Materials for Interlayer Dielectric Applications," Solid StateTech., pp. 83-90, March 1996; Singer, "Low k Dielectrics: The SearchContinues," Semiconductor Int'l., pp. 88-96, May 1996. Those articles,and all other articles, patents and patent applications cited in thisspecification are incorporated herein by reference.

Interlayer dielectrics according to the present invention aremicroporous, hybrid IOC materials. Such materials are formed frompreferably fluorinated inorganic and/or organic precursors and possesssufficient microporosity, as required, to achieve a dielectric constantof less than about 3. The hybrid IOC materials of the present inventioncontain an inorganic component linked to an organic matrix. Suchcomposites possess properties of both the inorganic component and theorganic component.

Including the inorganic component in the present compositesadvantageously imparts properties characteristic of the inorganic, e.g.,thermoresistance, permeation resistance, and the like, to the presentinterlayer dielectric. The organic polymeric component is characterizedby a low dielectric constant, typically in the range of 2 to 3. Thebenefits of the inherently low-dielectric characteristics of the organicpolymeric component are offset, however, by the comparatively higherdielectric attributes of the inorganic component.

In order to compensate for such higher dielectric attributes, hybridIOCs according to the present invention are microporous. Microporosityprovides such compensation because air, which has a much lowerdielectric constant than that of the inorganic or organic components, isretained within pores. Since the dielectric properties of the completehybrid IOC are averaged over its constituent parts, the presence of airlowers average dielectric attributes. Additionally, the inorganiccomponent or the organic component, or more preferably, both, containfluorine. As is known in the art, fluorine reduces the dielectricconstant of systems associated therewith. Thus, the present interlayerdielectrics are characterized by three phases; an inorganic phase, anorganic phase and a void phase.

It should be noted that the phrases "inorganic component," "inorganicphase" and "inorganic particles" are used interchangably herein. Asdescribed in more detail below, the inorganic component, etc., isgenerated, typically via oligomerization, from a precursor component (orphase or particle) that has an inorganic constituent but which may notbe purely "inorganic." That is, the inorganic component precursor mayinclude an organic constituent, as well. Similarly, the phrases "organicphase," "organic component" and "organic matrix" are usedinterchangably. The organic component, etc., is generated, typically viapolymerization, from an organic component (or phase) precursor. Theterms "phase," "component" or particles" will be used to refer to boththe component and its precursor unless otherwise noted. Moreover, whenreferring to the present invention, the terms "interlayer dielectric"and "hybrid IOC" are synonomous.

As noted under the Summary of the Invention, steps in a method forfabricating the present interlayer dielectric materials should beconsidered to be arbitrarily ordered and substantially permutable. Thatis, they may carried out in a different order, unless noted to thecontrary. One notable exception, previously mentioned, is that theorganic component precursor is polymerized to form the organic componentafter it is deposited on a substrate. Once the organic component isformed, the materials composition is not spinnable.

FIG. 1 shows an exemplary embodiment of a method according to thepresent invention. Described in terms of its constituent operations, thepresent method includes a first operation 102 of forming a three-phasecomposite precursor formulation, a second operation 104 of depositingthe precursor formulation onto a substrate, and a third operation 106 ofreacting the deposited formulation to synthesize the present interlayerdielectric. Those three operations are described in detail below inconjunction with FIGS. 2-4.

FIG. 2 shows an exemplary embodiment of a method for forming thethree-phase composite precursor formulation, i.e., operation 102 ofFIG. 1. As indicated in operation block 202, an inorganic component,i.e., a precursor or the component, having an organic functionality OFis provided. As described later in this specification, the organicfunctionality OF will be used to couple the inorganic component to theorganic component.

The inorganic component containing the organic functionality OF isprepared, in a first embodiment, by linking the inorganic component witha coupling agent possessing the organic functionality OF. Such a link isillustrated conceptually in FIG. 3. The coupling agent 302, whichpossesses the organic functionality OF and also possesses an inorganicfunctionality IF, is linked to inorganic component 308. The link isestablished via a preferential interaction between the inorganicfunctionality IF present in the coupling agent and a functional group orgroups 310 present in the inorganic component 308.

Before describing additional steps in the formation of the precursorformulation, further description of the nature of the inorganicprecursor, inorganic component and the coupling agent is provided.

In presently preferred embodiments, the inorganic component used forforming the present compositions are synthesized from an inorganiccomponent precursor. The inorganic component precursor is anorganometallic compound, typically a metal alkoxide. Metal alkoxide canbe represented by the formula M(OR)₄ : ##STR1## where: --OR is analkoxide group, which can be individually selected, and M is a metal,preferably germanium (Ge), titanium (Ti), zirconium (Zr) or tin (Sn) andmore preferably silicon (Si).

Several non-limiting examples of such metal alkoxides include tetraethylorthosilicate (TEOS), tetramethyl orthosilicate (TMOS), zirconium (IV)butoxide and zirconium (IV) propoxide. There are no per se limitationson the size of alkyl group R. As the size of R increases, however, therate at which the inorganic component is formed via loss of the --ORgroups decreases. As a practical matter, smaller sized alkyl groups arepreferred. Metal alkoxides suitable for use in conjunction with thepresent invention can be purchased from a manufacturer, such as Gelest,Inc., of Tullytown, Pa. Alternatively, the metal alkoxide can besynthesized according to well known methods.

In other embodiments, modified-metal alkoxides can be used as aprecursor. In such modified-metal alkoxides, less than all, andpreferably one, of the --OR groups of the metal alkoxide are replaced byan --R¹ group, i.e., R¹ _(n) M(OR)_(4-n). During synthesis of theinorganic component, alkoxide groups, i.e., --OR, are driven off asvolatile byproduct that is not retained in the final inorganiccomponent. The substituted group, i.e., the --R¹ group, however, isretained in the final inorganic component. The --R¹ group is an organicgroup, i.e., carbon containing, that may be polymerizable ornon-polymerizable. Note that even though the reaction product from themodified-metal alkoxide R¹ _(n) M(OR)_(4-n) is not properly categorizedas "inorganic," the sol-gel reaction product with the retained --R¹ willstill be referred to as the inorganic component, etc.Methyltriethoxysilane (MTEOS) is an example of such a modified alkoxide.See Brinker et al., Sol-Gel Science: The Physics and Chemistry ofSol-Gel Processing, p. 115, (Academic Press, CA 1990).

Advantageously, one or more of the --OR groups of the metal alkoxide isreplaced with a fluorine-containing --R¹ group. A nonlimiting example ofa compound resulting from such substitution is(tridecafluoro-1,1,2,2-tetra-hydrooctyl)triethoxysilane: ##STR2## Suchfluorine-containing metal alkoxides are available from Gelest, Inc., ofTullytown, Pa.

As previously described, the inorganic component has a dielectricconstant-raising effect on composites according to the presentinvention. Thus, it is desirable to limit the concentration of theinorganic component in the composite to the amount required forimparting the desired characteristic attributes, i.e, thermal resistanceand the like. It will be appreciated that such a concentration will varywith the organic component selected, since a given organic componentwill contribute its own characteristic attributes to the composite. Assuch, the concentration of the inorganic component is best determined byroutine experimentation as is within the capabilities of those havingordinary skill in the art.

In general, however, the inorganic component may be present in thehybrid inorganic organic composite in an amount in the range of fromabout 1 to 50 weight percent based the combined weight of the inorganicand the organic component. In other words, at a maximum, the inorganiccomponent can be present in about a 1:1 ratio with the organiccomponent. More preferably, the inorganic component is present in aconcentration less than about 30 weight percent based on the combinedweight of the inorganic and the organic component. Thus, the inorganiccomponent precursor is provided in an amount within the ranges statedabove.

The inorganic component is generated in-situ from the inorganiccomponent precursor via controlled nucleation and growth methods. Theinorganic component is formed in two steps that occur simultaneouslyafter initialization. The first step is monomer formation via partialhydrolysis of the inorganic component precursor, which is typically themetal or modified-metal alkoxide, e.g.:

    R.sup.1.sub.n M(OR).sub.4-n +H.sub.2 O→R.sup.1.sub.n (RO).sub.4-n-1 MOH+ROH                                                   [2]

Solvent and catalyst can be used to promote the reaction. The secondstep is polycondensation of the monomers to form colloid-like oligomers:##STR3##

Thus, the inorganic component "grows" via a controlled polymerization.Though only figuratively accurate, it is convenient to refer to theinorganic component as a collection of "particles." The inorganicparticles are perhaps best described as regions of relatively uniformdensity that are rich in the inorganic component. The polymerization canbe controlled to yield particles having a specific molecular weight,i.e., size. The particles forming the colloid are, on average, nanometer(nm)-sized. That is, the particles have an average particle size of lessthan a micron in diameter, and are preferably less than 400 nm indiameter. The formation of such nanometer-sized in-situ-generatedinorganic particles is well known in the art. See, for example, Yoshida,A., "Silica Nucleation, Polymerization and Growth Preparation ofMono-Disperse Sols," in The Colloidal Chemistry of Silica, Bergna H. E.,ed., Adv. Chem. Ser. 234, ACS, Wash. D.C. (1990); Gelest Catalog forSilicon, Germanium, Tin and Lead Compounds, Metal Alkoxides, Diketonatesand Carboxylates, pp. 279-283 (1995). Those skilled in the art willrecognize that the chemistry described above applies well known sol-geltechniques.

Though less preferred, a suspension of preformed nanometer-sized(average) inorganic particles can be used as an alternative togenerating the inorganic particles in-situ as described above. Usingsuch preformed particles is less preferred because during conventionalblending, i.e., melt blending, particle agglomeration typically results.Such agglomeration may lead to poor mechanical properties if thesize-scale of heterogeneity is sufficiently high.

Most multi-component systems suffer failure at the component interface.Thus, it is desirable to increase inter-component adhesion in somemanner. This is the function served by the coupling agent. It waspreviously noted that the coupling agent acts as a bridge to link theinorganic component to the organic component by forming associationswith each. The coupling agent may alternatively be conceptualized asgenerating an "interphase region." Such an architecture should enhancethe mechanical properties of the hybrid IOC-based interlayer dielectricby facilitating inter-component stress transfer. Using a coupling agentalso reduces inorganic phase agglomeration and phase separation.Furthermore, it has been observed that the coupling agent can result inenhanced thermal resistance.

Joining the inorganic and organic components to the coupling agent hasbeen described, alternatively, as linking, coupling, and associating.The interaction between the organic and inorganic functionalities OF, IFof the coupling agent with the groups present in the organic andinorganic components, e.g., group 310 of the inorganic component, isbelieved to be a covalent bond-forming reaction. Those having ordinaryskill in the art will be able to select a suitable coupling agent basedon the inorganic and organic components being used.

For example, if the organic component precursor is a polyimide precursorand the in organic component precursor is(tridecafluoro-1,1,2,2-tetra-hydrooctyl)triethoxysilane are used, asuitable coupling agent contains a primary amine functionality, e.g.,R--NH₂, and Si--O--R groups. The amine functional group reacts with thepolyimide precursor forming a polyamic acid amide linkage beforeimidization of the precursor. The Si--O--R groups, where R is anynonpolymerizable alkyl group, such as, without limitation, a methyl oran ethyl group, react with the silanol (Si--O--H) groups resulting from(tridecafluoro-1,1,2,2-tetra-hydrooctyl)triethoxysilane hydrolysis.Preferably, the coupling agent contains at least two --OR groups boundto a single metal atom, e.g., silicon for the above example. Given theabove organic and inorganic components, suitable exemplary couplingagents include, without limitation, 3-aminopropyltriethoxysilane,3-aminopropyltrimethoxysilane,3-aminopropyltris(methoxyethoxy-ethoxy)silane,3-aminopropyltris(trimethylsiloxy)-silane. In preferred embodiments, theaformentioned coupling agents are fluorine-substituted.

The aforementioned exemplary coupling agents have only a single primaryamine group, and will function as a chain terminator, limiting the chainlength of the organic component grafted onto the inorganic component. Inpreferred embodiments, the coupling agent has two primary amine groupsso that it will function as a chain extender. Coupling agents having twosuch primary amine groups, however, are not readily commerciallyavailable. Such coupling agents can, however, be synthesized. See, forexample, Morikawa et al., "Preparation of New Polyimide-Silica HybridMaterials via the Sol-Gel Process," J. Mater. Chem., V.2, no. 7, pp.679-90 (1992).

It will be appreciated by those skilled in the art that the R group ofthe inorganic functionality IF of the coupling agent will affect therate and extent of the reaction between the coupling agent and theinorganic component. Furthermore, it should be understood that inpreferred embodiments, the coupling agent and the inorganic compoundcontain the same metal. Coupling agents can be obtained frommanufacturers such as Gelest, Inc. of Tullytown, Pa. See, for example,Gelest Catalog for Silicon, Germanium, Tin and Lead Compounds, MetalAlkoxides, Diketonates and Carboxylates, pp. 41-52 (1995).

The coupling agent should be added to the inorganic component in anamount sufficient to bond to some portion of accessible M--O--H groupsof the inorganic particles. The amount of accessible M--O--H groups isestimated by calculating a total number of surface M--O--H groups basedon average particle size. A theoretical coupling agent requirement isthen estimated based on the number of such surface M--O--H groups. Anexcess of two to three times the theoretical requirement may suitably beadded to the inorganic component.

It should be appreciated that the theoretical coupling agent requirementmay vary with the particular inorganic compound selected and itsconcentration. Coupling agent requirements are best determined, however,by routine experimentation. A figure of about 0.3 has been found to besatisfactory for the molar ratio of (the coupling agent):(the metal inthe organic compound precursor) for the system described in the Examplelater in this specification. It is expected, however, that lesseramounts of coupling agent may suitably be used. For example, it isbelieved that bonding a minor portion, i.e., less than 50 percent, ofaccessible M--O--H groups is acceptable if the minor portion is welldistributed about the "perimeter" of the particle.

In certain embodiments, the coupling agent oligomerizes. Conditions areselected to reduce oligomerization to promote efficient use ofmaterials. For example, reducing coupling agent concentration reducesoligomerization. Such conditions are known to those skilled in the art.See Keefer, K. D., in Silicon-Based Polymer Science, ACS Symp. Ser., v.224 (1990).

It will be appreciated by those skilled in the art that theaforementioned coupling agents are modified-metal alkoxides having atleast one --R¹ group that can participate in organic polymerizationreactions. As previously described, such modified-metal alkoxides arealso suitable for forming the inorganic component. Hence, in a secondembodiment illustrated conceptually in FIG. 4, an inorganic component406 having the organic functionality OF (required in block 202 of FIG.2) is generated from a coupling agent 402 containing the organicfunctionality OF. In such embodiments, the coupling agent serves as theinorganic component precursor, and the previously-described ranges forthe inorganic component precursor concentration apply. Again, theorganic functionality OF of the coupling agent is selected based on theorganic component precursor.

In embodiments in which the coupling agent is the inorganic componentprecursor, it is not necessary to include additional coupling agent tograft the coupling agent-based inorganic component to the organiccomponent since the organic functional group from the coupling agent isretained in the inorganic component formed therefrom. It should beunderstood that the coupling agent is preferably fluorine substituted ifthe organic component precursor does not contain fluorine.

With continuing reference to operation 202 of FIG. 2 (providing aninorganic component having an organic functionality as a step in forminga three-phase composite precursor formulation), linking the inorganiccomponent to the coupling agent is not required at this point in themethod. In other words, operation 202 is to be understood to requireproviding, as a minimum, the constituents required for synthesizing aninorganic component having an organic functionality.

In operation 204, an organic component precursor andniicroporosity-imparting agent are added to the constituents required byoperation 202. It should be understood that the order of operations 202and 204 are reversible; the inorganic component can be formed with theorganic compound precursor and the microporosity-imparting agentpresent.

Thus, by operations 202 and 204, a three-phase composite precursorformulation is formed. The formulation is referred to as a "precursor"formulation, since, at this point in the method, the organic matrix isnot fully developed, but is present in the form of the organic componentprecursor. Likewise, the third phase, which is the void phase, i.e, themicroporosity generated within the organic matrix, is also not yetdeveloped.

It was indicated above that the order in which various linking andreaction steps are carried out is substantially freely permutable. Assuch, it should be appreciated that with such permutations, thecomposition of the three-phase composite precursor formulation changes.For example, in one embodiment, the formulation includes an inorganiccomponent precursor, an unlinked coupling agent, an organic componentprecursor and a micro-porosity imparting agent. In a second embodiment,the formulation includes an inorganic component that has beensynthesized from the inorganic component precursor. In a thirdembodiment, the inorganic component has been linked to the couplingagent. In a fourth embodiment, the coupling agent is linked to theorganic component precursor before it is linked to the inorganiccomponent. In a fifth embodiment, the coupling agent is itself theinorganic component precursor so additional coupling agent is notrequired beyond that required for synthesizing the inorganic component.

As previously described, it is advantageous for an interlayer dielectricmaterial according to the present invention to contain only that amountof the inorganic component that is required to impart the desiredphysico-chemical properties. Increasing the concentration of theinorganic component beyond that amount disadvantageously andunnecessarily increases the dielectric attributes of the interlayerdielectric. Thus, it is preferable to select an organic precursor thatis known to produce a polymer possessing a greater measure of thermalstability and other desired physico-chemical properties. Such preferredprecursors include, without limitation, those that form ring-containingpolymers, either aliphatic or aromatic rings. In a particularlypreferred embodiment, the organic component is an aromatic polyimide. Asis known to those skilled in the art, aromatic polyimides can besynthesized by the reactions of dianhydrides with diamines ordiisocyanates. See, Odian, Principles of Polymerization, (John Wiley &Sons, 3d. ed.). Once an organic component precursor is selected, aparametric study is preferably undertaken wherein various hybrid IOCsare formed using the selected precursor and various levels of a selectedinorganic component. The thermogravimetric properties, the coefficientof thermal expansion and other physical properties of the varioussamples are measured. A hybrid IOC for use as an interlayer dielectricis then suitably selected based on its dielectric constant andphysico-chemical properties.

Returning to FIG. 1, in operation block 104, the three-phase compositeprecursor formulation is deposited as a layer having a suitablethickness and appropriate gap-fill and step-coverage characteristics ona surface, such as a silicon wafer. In presently preferred embodiments,the formulation is deposited by spin-coating. In other embodiments,derivatives of plasma or chemical vapor deposition may suitably be used.Optional formulation additives, such as, without limitation, a viscositymodifier, can be mixed into formulation near to the time of deposition.Viscosity modifiers can be used to reduce or increase formulationviscosity, if necessary. If formulation viscosity is too high, forexample, it is difficult to obtain a uniform film. Volatile solvents,among other modifiers, can be used to lower the formulation viscosityfor spin coating. Such volatile solvents evaporate during spin coatingand from the deposited film thereafter. The spin-coating formulation canbe from low molecular weight monomers, as in Examples I-III presentedlater in this specification, from oligomer/pre-polymer mixtures, or fromsolution wherein the formulation is "dissolved" in "solvent."

For future generation devices having 0.25 micron line width and less,the precursor is deposited so as to yield a film thickness of a 1 micronor less. Moreover, the precursor formulation should possesscharacteristics enabling optimal gap-fill and step-coverage. Selectionof appropriate spin-deposition process conditions, e.g., spin-rate,spin-time; deposition temperature; formulation viscosity and the like,for achieving such characteristics are within the capabilities of thosehaving ordinary skill in the art.

In block 106, after the three-phase composite precursor formulation isdeposited on a wafer or the like, it is thermally processed. Thermalprocessing volatizes/evaporates any solvent or dispersing fluid, inaddition to promoting chemical reaction to convert the depositedprecursor formulation to its ultimate chemical and physical state. Inparticular, the organic component precursor is polymerized forming theorganic matrix, and the inorganic component is linked thereto. Thermalprocessing can be performed in air, or, alternatively, under nitrogenatmosphere to minimize the possibility of oxidative degradation.Anticipated processing conditions for a spun-on polyimide-based systemaccording to the present invention includes a 15 minute bake at 250° C.and a 1.5 hour ramp up to 400°.

The microporosity-imparting agent is either reacted or incorporatedwithin the polymerizing organic phase precursor, depending on the natureof the agent, generating void space. In particular, microporosity isimparted, in a first embodiment, by using foaming agents, in a secondembodiment using thermally labile constituents, and in a thirdembodiment by incorporating molecules exhibiting porous caged-structuresuch as Buckminster fullerenes or zeolites. A description of theaforementioned microporosity-imparting agents follow.

Regarding the first embodiment, foaming agents undergo chemical reactionforming volatile gases above a known, definite temperature. In thepresent method, the foaming agent undergoes chemical reaction duringpolymerization of the organic precursors. The subsequent liquid-to-gasexpansion and gaseous evolution from the system caused by the foamingagent generates void space or porosity within the surrounding organicmatrix.

The size and number of pores, i.e., pore density, and degree of porecontinuity or interconnectivity can be empirically controlled byregulating the matrix composition and viscosity, the concentration andparticle size of the gas-generating material, and the thermal processinghistory utilized.

As to concentration, higher concentrations of the foaming agent willincrease the concentration of pores/void space. Regarding particle size,the foaming agent is typically supplied as a granular solid powder, andparticle size affects the number and the size of the pores generated.Particle size affects the number of pores created since gas is evolvedfrom each particle of the foaming agent. If the particles are smaller,then there will be more of such particles at a given concentration offoaming agent. Consequently, more pores are generated. Pore size iscontrolled by the volume of gas liberated, which is proportional toparticle size. Thus, while decreasing particle size for a fixed amountof foaming agent increases the number of pores, it decreases the averagepore size.

As noted above, the specifics of the thermal processing cycle alsoaffects the degree of porosity; however, that relationship iscomplicated and more readily studied via empirical analysis. Inprinciple, higher temperatures should increase porosity due to decreasedviscosity. Since, however, the organic matrix will undergothermally-driven cure which raises viscosity, the overall effect onporosity is difficult to predict.

Exemplary suitable foaming agents include, without limitation, variousoxides, peroxides, halides, hydrates, carbonates, sulfates, sulfides,nitrides and nitrates, and compounds of certain elements such as Al₂(SO₄)₃, Co₃ O₄, PbCl₂, CuO, Mn₂ O₃, WO₃, BaO₂, and the like. Preferably,the foaming agent is IBr, InCl₃, Fe₂ Br₆ and MnCl₂ or other likecompounds that are active at temperatures below about 350° C. Regardingthe second embodiment, a thermally-labile constituent is homogenouslydispersed in, and reacts with, the organic component precursor. Theconstituent is itself a precursor that generates polymeric materialsthat react with the organic component forming regions locally-rich inthe generated polymeric material. Such polymers readily depolymerize or"chain-unzip" reforming the corresponding monomers/chain fragments attemperatures above the ceiling temperature, t_(c), defined as thetemperature at which the polymerization reaction becomes reversible.Such depolymerization causes voids or porosity in the organic matrix. Itwill be appreciated that the ceiling temperature of the polymer formedfrom the thermally-labile component must be lower than the temperatureat which the other components, i.e., the inorganic particles, couplingagent, or polymer matrix, will suffer deleterious effects.

Thermally-labile constituents and systems are well understood by thoseskilled in the art, and include, without limitation,poly(methylmethacrylate) for which t_(c) =220° C.; poly(α-methylstyrene) for which t_(c) =61° C.; and poly(isobutylene) for which t_(c)=50° C.

In the third embodiment, a molecule exhibiting a porous caged-structure,such as a Buckminster fullerene, or a zeolite, can be incorporated intothe interlayer dielectric material. Due to their caged-structure, suchmolecules will impart a porous microstructure to the interlayerdielectric. Unlike the previous two embodiments wherein the thermallylabile constituent or foaming agent leave voids in the material, theselected molecule will remain in the interlayer dielectric. As such, itmay be derivatized with a functional group capable of chemicallyreacting with the functional group in the organic compound. Methods forderivatizing such systems are well known to those skilled in the art.See, Diederich & Thilgen, "Covalent Fullerene Chemistry," Science, v.271, p. 317 (1996); Dresselhaus et al., Science of Fullerenes and CarbonNanotubes, Academic Press (1996). Zeolites suitable for use inconjunction with the present invention have a pore size of less thanabout 12-13 angstroms, which includes most if not all zeolites. See,Breck, Zeolite Molecular Sieves, (Krieger Publishing Co., 1984 reprint).

It is known that a serious drawback of the sol-gel methodology is dryingshrinkage. Drying shrinkage occurs as cosolvents and reaction byproductsare removed from the sol-gel solution. Shrinkage causes cracks in thematerial. Such shrinkage can be avoided if all the initial componentsand byproducts can be incorporated directly into the resulting polymer.Thus, in a preferred embodiment, a means for reducing drying shrinkageis added. Such means can be a polymerizable solvent. Preferably, thepolymerizable solvent is the organic component precursor itselfHydroxy-terminated monomers can be suitable for this purpose. See, Novaket al., "Simultaneous Interpenetrating Networks of Inorganic Glasses andOrganic Polymers: New Routes into Nonshrinking Sol-Gel DerivedComposites," Polym. Prep., v.31, pp. 698-99 (1990); Ellsworth et al.,"Mutually Interpenetrating Inorganic-Organic Network: New Routes intoNonshrinking Sol-Gel Composite Materials," J. Am. Chem. Soc., v. 113,pp. 2756-58 (1991).

In preferred embodiments, the organic component-forming polymerizationreaction is initiated by a free-radical initiator. In a furtherpreferred embodiment, an organic phase cross-linking agent is added aswell. The free-radical initiator and the cross linker are selected basedon the organic precursor being used. Such selection is within thecapabilities of those skilled in the art. Free-radical initiator andcross linker requirements based on organic precursor feed is in therange of from about 1 to about 5 weight percent.

In the following non-limiting examples, the foregoing method is appliedto generate three-phase composite precursor formulations that can bespin-coated and polymerized to form materials suitable for use asinterlayer dielectrics. It will be appreciated that other inorganic andorganic precursor components can be utilized in conjunction with thepresent method for generating three-phase hybrid IOCs suitable asinterlayer dielectric materials.

EXAMPLE I Forming A Three-Phase Composite Precursor FormulationEmbodiment 1

A hybrid IOC system according to the present invention having athree-phase microstructure was formed from silica as the inorganiccomponent and 2-hydroxyethyl methacrylate as the organic component asfollows. The inorganic component was generated from sodium metasilicate,an inexpensive, commercially available precursor, as described below. 17grams of solium metasilicate was dissolved at ambient temperature (about25° C.) in 0.1 liters of de-ionized water. A solution of 3.0 M HCL wasprepared and 0.1 liters of such solution was transferred to a 500 cm³3-neck flask and permitted to equilibrate at 0° C. in an ice-bath.Purified nitrogen was bubbled through the HCL solution for approximately10 minutes. The sodium metasilicate solution was then added dropwise tothe HCL solution with continuous mixing. After complete addition of thesodium metasilicate solution, the resulting solution was left to stirfor three hours. The flask was then removed from the ice-bath, 60.0 g.of NaCl, 0.02 liters of NaOH and 0.16 liters of tetrahydrofuran (THF)were added and the system was stirred vigorously for one hour. Theresulting suspension was filtered, and the filtrate was collected andtransferred to a 0.5 liter separatory funnel. The aqueous layer wasseparated and discarded while the organic layer containing synthesizedparticulate silica, i.e., oligomers of poly(silicic acid), wascollected. Purified nitrogen was bubbled through the solution.

The molecular weight of the poly(silicic acid) obtained is controlled bythe reaction time and temperature. Experimental conditions for obtainingspecific molecular weights are known to those skilled in the art. See,Abe et al., "Preparation of Polysiloxanes from Silicic Acid III:Preparation and Properties of Polysilicic Acid Butyl Esters," J. PolymerSci., 21(41), 1983; Ellsworth et al., "Inverse Organic-InorganicComposite Materials 3: High Glass Content Non-Shrinking Sol-GelComposites via Poly(silicic) acid esters," Chem. Mater., vol. 5, p. 839,(1993); Yoshida, A., "Silica Nucleation, Polymerization and GrowthPreparation of Mono-Dispersed Sols," The Colloidal Chemistry of Silica,Bergna, H. E., ed., Adv. Chem. Ser. 234, ACS, Wash. D.C. (1990).

As discussed in more detail below, a methacrylate monomer, specifically2-hydroxyethylmethacrylate, was used as the organic component precursorfor forming the organic component. As such, a coupling agent havingvinyl functional groups to react with the vinyl functional group in themethacrylate monomer and having alkoxysilane groups to react with thesilanols of the poly(silicic acid) was selected.Methacryloxypropylmethyl-dimethoxysilane, which possesses suchfunctional groups, was selected as a coupling agent.

0.01 liters of coupling agent and 0.01 liters of 3.0 M HCL were added tothe colloidal silica solution. The system was stirred continuously fortwo hours at 25° C. Those conditions were selected to minimizeoligomerization of the coupling agent. Next, 50 g. of NaCl and 0.1 literof de-ionized water were added. The resulting solution was stirred foran hour and then filtered. The filtrate was transferred to a separatoryfunnel and the organic portion recovered. 30.0 g. of anhydrous sodiumsulfate was added, stirred for four hours, and then removed viafiltration. 8.36 g. of 2-hydroxyethyl methacrylate monomer was added tothe organic portion to obtain a material ultimately incorporatingorganic and inorganic material in a 1:1 ratio on a weight basis. Aspreviously described, lesser amounts of the inorganic component, such asbetween 1-30 weight percent may suitably be added to moderate thedielectric attributes incorporated into the resulting IOC. It should beunderstood that the organic monomer was not polymerized at this time.Polymerization is initiated only after depositing the formulation ontoan intended surface, e.g., silicon wafer, etc., in a later step.

Purified hydrogen was bubbled through the solution to promotefree-radical inhibition, minimizing organic-phase polymerization. Thesystem was then heated via immersion in a water bath that was maintainedat 80° C. Heating continued until approximately 0.01 liters of liquid(the solvent) was distilled off and collected. The residue was recoveredand 0.17 g. each, i.e., 2 weight percent based on the monomer added, ofa free radical inhibitor (benzoyl peroxide), and a trifunctionalacrylate cross linker (2-ethyl-2-hydroxymethyl-1,2 propandioltriacrylate) were added. Residual THF was removed via a rotaryevaporator.

A microporosity-imparting agent was added to the formulation and evenlydispersed. The agent was a monomer which, on polymerization, formsregions within the composite structure that are locally rich in athermally-labile organic polymeric component. The thermally-labileconstituent within these regions is then depolymerized or"chain-unzipped" via a thermally-initiated chemical-decompositionreaction, which takes place during monomer polymerization to form theorganic matrix. The decomposition reaction generates voids in thematerials architecture where such decomposition occurs.

The microporosity-imparting monomer used was a-methyl styrene monomer.The corresponding polymer generated from this monomer readily undergoesdepolymerization at temperatures above about 61° C. The α-methyl styrenemonomer was added in an amount equivalent to five percent by weight withrespect to the organic component precursor. Of course, other monomersmay suitably be used to generate other thermally-labile components.

EXAMPLE II Forming A Three-Phase Composite Precursor FormulationEmbodiment 2

The same inorganic component and organic component are used as inExample I, but the microporosity-imparting agent is different. Inparticular, microporosity is to be imparted in the present example via a"foaming agent," which, as the term is used herein, is a compound thatundergoes thermally-initiated chemical reaction to form volatile gas. Asubsequent material expansion and gaseous evolution occurs generating a"foaming" effect within the material superstructure that forms voidspace or microporosity. In the present example, IBr is used as thefoaming agent. Two percent by weight of the foaming agent, with respectto the organic precursor, is added to the formulation in powder form(ave. particle size ca. 2 microns) and evenly dispersed.

EXAMPLE III Forming A Three-Phase Composite Precursor FormulationEmbodiment 3

The same inorganic component and organic component are used as inExamples I & II, but the microporosity-imparting agent is different.Specifically, microporosity is obtained by incorporating moleculeshaving caged, porous structures. Accordingly, pore size of the resultinghybrid IOC is controlled directly via the pore size of the incorporatedmolecular constituent. In the present example, commercially-availableBuckminsterfullerene C-60 (powder form), available from Aldrich ChemicalCo., is added to the formulation and evenly dispersed. Two percent byweight of the Buckminsterfullerene, with respect to the monomer, isadded.

The three-phase composite precursor formulations generated in ExamplesI, II and III may then be deposited, such as by spin-coating, ontosilicon wafers containing a layer of thermal oxide. The precursorformulation obtained in Example I was spin coated on to a wafer at aspin rate in the range of about 2200 to 3000 rpm for 20 seconds at 25°C. Those conditions are suitable for obtaining an average nominaldeposited film thickness of less than about 1.5 microns. As previouslynoted, a thinner film, i.e., about 1 micron or less, should be used ifthe interlayer dielectric is to be patterned in 0.25 micron or lessline-widths. The deposited material is then subjected to thermalprocessing in order to polymerize the organic matrix phase and generatethe final three-phase composite microstructure.

For the systems described in Examples I, II and III, organicpolymerization can be initiated by thermal treatment under nitrogen inan oven at 60° C. for eight hours. The system is then exposed to thermaltreatment at higher temperature, e.g., greater than 180° C. for 10minutes and then held at 120° C. under vacuum for 12 more hours. Itshould be understood that the aforementioned treatment times can bereduced significantly by increasing processing temperature and/orutilizing a partially-reacted organic precursor, e.g., an oligomer.

The 2-hydroxyethyl methacrylate monomer used in the Examples forms apolymer having a relatively high dielectric constant. Other monomerscapable of producing an organic material exhibiting a lower dielectricconstant, higher glass transistion temperature and greater thermalstability than 2-hydroxyethyl methacrylate monomer are presentlypreferred for forming three-phase precursor formulations according tothe present invention. Such preferred formulations are more suitable foruse in forming ILDs to be utilized in future-generation microelectronicdevices. Preferred formulations utilize, for example, monomer systemssuitable for forming aromatic polyimides. Such monomer systems includepyromellitic anhydride with p-phenylene diamine or fluorinated analogsof these molecules. Additionally, other systems having aromaticstructures and/or fluorinated groups situated in the polymer repeat-unit(either the main/backbone chain or side-group) should also result inhigh stability, low dielectric constant materials.

Interlayer dielectric material formed according to the present inventionis suitable for use in future-generation microelectronics devices, suchas, for example, logic devices, memory devices and the like. It isexpected that standard procedures for forming such devices can be used.See, for example, Wolf and Tauber, Silicon Processing for the VLSI Era,vol. 1-3, (Lattice Press, 1990). As such, the three-phase compositematerials systems disclosed herein will be subjected to variousoperations during device manufacture, particularly for multi-layerapplications. Such operations include planarization, e.g., "dry-etch"plasma processing or chemical/mechanical polishing; etching; patterning,e.g., reactive-ion-etching; metallization, e.g., CVD metal deposition;and thermal processing operations, e.g., post-etch annealing, chipbonding/packaging.

Although specific embodiments of this invention have been describedherein, it is to be understood that these embodiments are merelyillustrative of the principles of this invention. Numerous and variedmodifications may occur to, and be implemented by, those of ordinaryskill in the art in view of the present teachings without departing fromthe scope and the spirit of the invention.

I claim:
 1. A method for forming a three-phase composite precursorformulation suitable for forming an interlayer dielectric, comprisingthe step of:forming a mixture of (i) an organic-component precursorpresent in an amount in the range of about 40 to 90 weight percent ofthe mixture; (ii) an inorganic-component precursor or a reaction productthereof having an organic functionality, wherein the organicfunctionality is selected for its ability to form a link to theorganic-component precursor or a reaction product thereof, and furtherwherein the inorganic-component precursor or its reaction product ispresent in an amount in the range of between about 1 to 50 weightpercent based on the combined amount of the inorganic-componentprecursor and the organic-component precursor in the mixture; and (iii)a microporosity-generating agent.
 2. The method of claim 1, and furtherwherein the organic functionality is provided by combining theinorganic-component precursor or the reaction product thereof with acoupling agent.
 3. The method of claim 2, and further wherein thecoupling agent has an inorganic functionality, wherein the inorganicfunctionality is selected for its ability to form a link to theinorganic-component precursor or the reaction product thereof.
 4. Themethod of claim 1, and further wherein the inorganic-component precursoris an organometallic compound.
 5. The method of claim 4, and furtherwherein the organometallic compound is selected from the groupconsisting of metal alkoxides and modified-metal alkoxides.
 6. Themethod of claim 5, and further wherein the modified-metal alkoxidecontains fluorine.
 7. The method of claim 1, and further wherein thereaction product of the inorganic-component precursor is an inorganiccomponent, wherein the inorganic component is a suspension of inorganicparticles containing a metal and characterized by an average particlesize of less than about one micron.
 8. The method of claim 7, andfurther wherein the organic functionality is provided by a couplingagent.
 9. The method of claim 8, and further wherein the coupling agentis present in an amount sufficient for coupling an amount of theinorganic component to a reaction product of the organic-componentprecursor.
 10. The method of claim 1, and further wherein theinorganic-component precursor or the reaction product thereof is areaction product of a coupling agent, wherein the organic functionalityis provided by the coupling agent.
 11. The method of claim 1, andfurther comprising the step of depositing the three-phase compositeprecursor formulation on a substrate.
 12. The method of claim 11, andfurther wherein the step of depositing comprises spin coating.
 13. Themethod of claim 11, and further comprising the step of generating athree-phase composite from the deposited three-phase precursorformulation.
 14. The method of claim 13, and further wherein the step ofgenerating comprises thermally processing the deposited three phaseprecursor formulation.
 15. The method of claim 14, and further whereinthermally processing the deposited three-phase precursor formulationcomprises the step of chemically reacting the organic-componentprecursor.
 16. The method of claim 15, and further comprising causingthe microporosity-generating agent to undergo a reaction that generatesmicroporosity while the organic-component precursor is chemicallyreacted.